39 



Working Distance. This term, strictly con- 

 sidered, is an invariable quality of the objective, and 

 is the distance between the front lens in the objective 

 and an uncovered object, when the objective is in 

 focus and is corrected for that object. All objectives 

 require a certain amount of projecting metal to pro- 

 tect the front lens, and this with a certain thickness 

 of the cover-glass lessens it. In objectives with fixed 

 mountings this may be, and with thick cover-glasses 

 is considerable. As it is comparatively unimportant, 

 however, for the working microscopist to know the 

 working distance per se of his objectives, but of con- 

 siderable moment to know what the actual space 

 between the objective and cover glass is, it would be 

 well, in the mind of the writer, to express it as avail- 

 able working distance. 



In objectives of low and medium power, it is of 

 little consideration ; but where it must be expressed 

 in -j-^. or TTr V IT inch, it becomes a matter of importance. 



Working distance is spoken of as being long or 

 short, and varies not so much with the power as with 

 the angular aperture ; generally the working distance 

 decreases with the increase in angular aperture, and 

 becomes greater as the aperture becomes smaller ; it 

 it was for a long time considered that these two prop- 

 erties varied according to a fixed rule, but this at the 

 present time is not considered to be the case. While 

 in objectives of the same aperture it may vary con- 

 siderably, it may in others of different aperture be so 

 that the higher one may have the greater working 



